The insualtion comes from trapped air.
Wool has natural oil on the fibres so that the water tends to bead on the surface leaving lots of air in the material.
With thinner cotton the water wicks straight through, giving nice thermal conducting water path from the skin to the outside.

It has to do with the fibers. Cotton wicks water and holds it in, and any air spaces fill up, so you stay wet and cold. Wool has kind of kinky fibers that make wool itchy and scratchy but maintain some air spaces even when wet. It's easier to wring water out of wool, too.

"Wool is a Hygroscopic Insulator
This means that wool can absorb moisture without becoming wet to touch. Wools natural insulation ability is not affected by moisture because of its built in natural crimp. As the fibres naturally repel each other, an amount of air is retained which provides an insulation effect."

Wool is hydrophobic - the surface energy of wool fibres is about 10x lower than cotton (wool is almost as good as polyprop).

Cotton fibres branch out into tiny feather like fibres that attract water, wool is smother, almost like polyester on a microscopic scale. I'm not sure how much the large shape of the fibres effect this - although it does mean the garment has more thickness to trap air.

But wicking is different to transferring water vapour so wool is also breathable.

Wool is hydrophobic - the surface energy of wool fibres is about 10x lower than cotton (wool is almost as good as polyprop).

I am not a scientist.

I've never heard of materials having surface energy before. The closest thing I can think of related to this is Einstein's equation E= Mc squared. In other words, I don't understand the signifigance of saying that the surface energy of wool fibers is 10x lower than cotton. I don't understand how surface energy relates to insulation.

Cotton fibres branch out into tiny feather like fibres that attract water, wool is smother, almost like polyester on a microscopic scale. I'm not sure how much the large shape of the fibres effect this - although it does mean the garment has more thickness to trap air.

Only if there is only one layer of atoms to the fabric, which there isn't.

But wicking is different to transferring water vapour so wool is also breathable.

Surface energy means the energy of the bonds on the surface of a material.
Imagine a drop of water, each water molecule has a choice of wether to bond to other water molecules around it and form a droplet or instead to bond to the surface of the material it is sitting on and wet it.
Everythign wants to be in it's lowest energy state and since breakign the bonds to the other water molecules takes energy the water will only wet a surface if the energy it will gain by making these new bonds is greater than the cost of breakign the ones inside the droplet. Water has very strong bonds to other water molecules so it will only wet materials with quite high surface energies.
That's why on most materials, like glass or metals, you see water as droplets.

If you have a material with a very high surface energy it is even possible for water to travel uphill to make those new energetic bonds - that's what happens when you put the edge of a piece of tissue paper in water and the water runs up the tissue to wet all of it - that's wicking or capilary action.

I don't thick that wool fibres repel each other - it's more that their kinked shape keep the fibres from lying flat together. This gives wool it's loft - why wool yarn is fluffy rather than thin and smooth. You can make wool into a smooth flat material without any trapped air (ie. felt) but it takes a lot of hammering to flatten out the fibres.

Molecular attraction / repulsion only works on very short distance - like a molecule sticking to a surface.

Does wool have a surface energy ten times lower than cotton because of the natural oil on the wool fibers?

mgb, In light of what you've told me, it seems to me that wool insulates when it's wet for the following three reasons: (1) the kink shape of wool fibers makes larger air pockets inside the fabric and (2) the natural oil on wool fibers makes water tend to bead up on the outside of wool rather than occupy the air pockets in wool and (3) wool has a lower surface energy than cotton.

I would think wool's surface energy is an intrinsic property of the surface rather than any oils - although adding oil as well will help.

The chemistry of wool and cotton is very different. Wool is a protein similair to the stuff that makes up your finger nails while cotton is a sugar although they look the same when made into clothers they are from opposite ends of biology.

Yes because the water will still tend to bead rather than wet the material ( due to surface energy)
The kinked wool fibres create more air spaces between fibres.
The wool garment generally has more thickness of material to trap air especially if it is knitted rather than woven.

mgb, if someone walked around in an extremely heavy rain for an hour wearing a wool sweater without the natural oils, after an hour wouldn't the wool sweater become saturated with water with the air spaces filling up with water too?

Probably (especially without the oils) although it is very difficult to saturate wool, even holding it under water it takes a lot of effort to get all the air out, it might take more than an hour to saturate! Walking makes it worse - the movement squeezes the water between the fibres.

With the oils - like a traditional fishermen's jersey it's hard to beat except for really good modern tech fabrics. Next to your skin as a base or mid-layer something like fine merino wool or wool-silk mix is still unbeatable by any modern fabric.

mgb, the reason that I'm asking these questions is I'm considering buying some wool pants to keep me warm this winter. I wanted to buy wool pants so my legs would still be insulated if I got wet while walking in the rain. When you told me about how the natural oils in the wool can be washed off the wool when one washes wool clothes in the wash machine, my reaction was to think that I should not buy wool pants because they would not have superior insulating properties compared to cotton pants after they got wet. In light of what you've told me however, it seems that wool pants will keep their insulating properties when wet indefinitely.

If all the air pockets in a wool sweater did somehow become saturated with water, couldn't a person just squeeze and twist the sweater to wring the water out of it and the air pockets would open up again?